Phosphorylation and dephosphorylation of the enzymes controlling the rate-limiting step is a most important regulatory mechanism for cellular metabolism. In diabetic animals the regulation of glycogen metabolism by this regulatory mechanism becomes defective and the defect can be corrected by the administration of insulin. The purposes of this work are: (1) to study the regulation of glycogen synthase and phosphorylase kinase activities by protein kinases and phosphoprotein phosphatases; (2) to define the defect in glycogen metabolism resulting from diabetes; and (3) to elucidate the mechanism of action of insulin.